Patent Application
20050237087
The present invention relates to current monitoring circuits, and, more particularly, to a low voltage current monitoring circuit that is independent of process, temperature and supply voltages even in high current and low voltage applications.
Power-supply current monitoring for testing of CMOS logic circuits monitors the current passing through the power supply VDD or ground GND terminals during the application of an input stimulus or while the circuit is in a quiescent condition.
Many of the existing current monitors, however, fail to provide reliable monitoring due to fluctuation in process, temperature, and voltage supply.
VCC=Vref(1+(R8+R2)/(R7+R3))
The current monitor 20 includes a sense FET 14, having a control node, a source node and a drain node, a resistor R6, a transistor Q1, a diode D1, and a resistor R4. The control node of sense FET 14 connects to amplifier 14. In operation, the current that goes through the sense FET 14 is divided down by n since the size of sense FET 14 is 1/n times the size of the main FET 16, where n is some integer value (i.e. 2, 3, 4, etc.). This same current flows across resistor R6 and generates a voltage that is equivalent to the base emitter voltage of transistor Q1. Once the voltage across resistor R6 is greater than the quiescent threshold voltage (−0.7V) of transistor Q1, transistor Q1 turns on. As a result, node P is pulled down and, thereby, the main FET 16 is turned off. Accordingly, excess current is prevented from flowing through main FET 16 after the threshold is reached.
Problems arise when the variations of process, temperature, voltage of the main FET 16, sense FET 14, and resistor R6 cause the voltages to vary and, thereby, creating voltage mismatches within the circuit. If the drain-to-source voltage VDS across main FET 16 and sense FET 14 do not match, the basic equation for the generating voltage VCC will be defeated.
VCC=Vref(1+(R10)/(R12))
Current monitor 60 includes a sense FET 62 coupled to an amplifier 64 that includes a feedback loop. Since the feedback loop exists, the voltage at node B1 is controlled. It is necessary to make certain that the voltage at node A1 equals the voltage at node B1. The current Ilim/n represents the feedback current ifdb that flows through resistor R16. The voltage at node C1 is represented in the following equation:
Vnode C1=Vsupply−R16(Ilim/n).
Amplifier 64 controls transistor 66 such that the current through resistors, R16 and R18, changes to make sure that the voltage at nodes A1 and B1 remain the same.
In operation, if the voltage at node B1 is greater than the voltage at node A1 by for example 100 mV, the gate voltage of transistor 66 will rise since the gate to source voltage will increase. Initially transistor 66 is in the saturation region, once the gate to source voltage Vgs increases, the feedback current ifdb will decrease to try to match and make the voltage at node A1 equivalent to that of node B1, such that the voltage at node B1 will decrease to equalize to that of node A1.
When the voltage at node A1 is greater than that of node B1, however, the current that flows through transistor 66 will decrease and the feedback current ifdb will increase to try to match and force transistor 66 into the saturation region. Thereby, the voltage at node B1 will increase to that of node A1.
Problems arise when the transistors process varies, thereby the voltage and current values will differ. In addition, when the temperature and supply voltage changes, this type of current monitor fails to provide a reliable determination due to drain-to-source voltage mismatch of main FET 54 and sense FET 62.
Thus, a need exists for a current monitor having a high performance, simple, and cost effective design that is independent of process, temperature and voltage.
The present invention is directed to overcoming, or at least reducing the effects of one or more of the problems set forth above.
To address the above-discussed deficiencies of current monitors, the present invention teaches a current monitor having a high performance, simple, and cost effective design that is independent of process, temperature and voltage. The current monitor includes a sensing transistor that couples to the main transistor of an adjoining voltage regulator. Specifically, the control and source nodes of each transistor couple to one another, respectively. The size of the main transistor is a predetermined multiple integer n of the size of the sensing transistor. A first resistor couples between a supply voltage and the drain node of the main transistor. A second resistor couples between a supply voltage and the drain node of the sensing transistor, wherein the size of the second resistor is equal to the size of the first resistor multiplied by the predetermined multiple integer n. An inverting input of an amplifier couples to the drain node of the sensing transistor, while a third resistor connects between the supply voltage and a non-inverting input of the amplifier. A control node of a transistor connects to the output of the amplifier. A drain node of the transistor feeds back to the noninverting input of the amplifier. A feedback resistor coupled between the source node of the transistor and ground. A current source coupled to the supply voltage. A first input of a comparator connects to the current source, while the second input of a comparator couples to the source node of the transistor. A reference resistor connects between the first input of the comparator and ground.
These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.
For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein:
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set for the herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
VCC=Vref(1+(R20)/R22))
Within current monitor 360, it is necessary that the voltage drop across resistor RS3 is equal to the voltage drop across resistor RS1. Since amplifier 342 coupled to transistor 344 forms a feedback loop, the voltage is controlled. It is necessary to make certain that the voltage at node A2 equals the voltage at node B2. The current IIlim/n represents the current that flows through resistor RS1. The voltage at node A2 is represented in the following equation:
Vnode A=Vsupply−RS1(Ilim/n).
The amplifier 342 controls transistor 344 such that the current through resistor RS2 changes to make sure that the voltage at nodes A2 and B2 remain the same.
A drain-to-source voltage VDS offset cancellation is implemented by placing a resistor RS3 in series with the main FET 312 which tracks the VDS between the main FET 312 and the sense FET 340. The size of sense FET 340 is a predetermined multiple n of the size of the main FET 312. Thereby, the size of resistor RS3 is equal to resistor RS1/n. The current that flows across resistor RS3 is Ilim, while the current that flows across resistor RS1 is Ilim/n. Thereby, even when the battery voltage varies, it will not affect the matching between the main FET 312 and the sense FET 340. It is important that the same amount of current must not flow through the sense FET 340 that flows through the main FET 312 or current will be wasted. The novel implementation decrements the current through the sense FET 340 by a factor of 1/n. This ratio will be constant with temperature, process, and voltage variation.
In operation, if the voltage at node B2 is greater than the voltage at node A2 by for example 100 mV, the gate voltage of transistor 344 will rise, since the gate to source voltage will increase. Initially transistor 344 is in the saturation region, once the gate-to-source voltage Vgs increases, the feedback current ifdb will decrease to try to match and make the voltage at node A2 equivalent to that of node B2, such that the voltage at node B2 will decrease to equalize that of node A2.
When the voltage at node A2 is greater that that of node B2, however, the current that flows through transistor 344 will decrease and the feedback current ifdb will increase to try to match and make the saturation region. Thereby, the voltage at node B2 will increase to that of node A2.
Those of skill in the art will recognize that the physical location of the elements illustrated in
Advantages of this design include but are not limited to a current monitor having a high performance, simple, and cost effective design that is independent of process, temperature and voltage.
The reader's attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.
All the features disclosed in this specification (including any accompany claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.
